The present invention relates to a system for controlling the air-fuel ratio of a mixture to a proper value during cold engine operation.
Feedback control systems for controlling the air-fuel ratio are known in the internal combustion engine emission control system art with a three-way catalyst. In one of such system, as in U.S. Pat. No. 4,132,199, an oxygen sensor for sensing the oxygen content of the exhaust gases and an electronic control circuit are provided for actuating an on-off type electromagnetic valve in dependency on the output voltage of the oxygen sensor to adjust the air-fuel ratio of the mixture to the stoichiometric air-fuel ratio. The output voltage of the oxygen sensor varies according to the temperature of the sensor device. More particularly, when the temperature of the sensor device is lower than a certain level, the output voltage is too low to operate the electronic control circuit for controlling the air-fuel ratio. Consequently, in such a cold condition, the feedback control system is rendered ineffective and driving pulses having a fixed pulse duty ratio is fed to the on-off type electromagnetic valve for providing a lean air-fuel mixture. On the other hand, an automatic choke device is provided to correct the lean air-fuel mixture to a proper air-fuel ratio according to the engine temperature for improving the operability of the cold engine.
The automatic choke device is adapted to close the choke valve by a spiral bimetal element in dependency on the cold engine temperature and to progressively open the choke valve as the temperature rises.
If however the speed of the engine is increased by opening the throttle valve in the choke valve closed condition, the amount of air induced into the engine increases. Accordingly the choke valve closed by the automatic choke device is irregularly opened or closed by the increased amount of air. As a result, the air-fuel mixture changes to an excessively lean or rich air-fuel ratio.